A number of patents disclose single-channel keyable control circuits. For example circuits disclosed in U.S. Pat. Nos. 3,624,415 and 3,628,099, both in the names of Carl E. Atkins and Arthur A. Cake, show keying circuits which require that the correct value of resistance in an external keying circuit be connected to actuate a keyable control circuit. In U.S. Pat. No. 3,723,967 in the names of Carl E. Atkins and Paul A. Carlson, a single channel inductively coupled tuned keying circuit absorbs energy from the radio frequency tank circuit of a free-running oscillator operating at the frequency to which the keying circuit is tuned. Radio frequency detection circuits detect the reduction in energy remaining in the oscillator and thereupon produce a control signal.
In U.S. Pat. No. 3,842,324 an external keying circuit includes a diode having a sharply variable junction capacitance with changes in diode bias as a component in a tuned circuit. When coupled to a keyable control circuit operating in the correct frequency range, absorbed rf energy causes rapid cyclic fluctuations in diode bias. The resulting rapid fluctuations in keying circuit resonant frequency alternately bring the keying circuit into and out of resonance with the rf frequency being generated. When in resonance, the keying circuit absorbs more rf energy from the rf oscillator than when out of resonance. The resulting amplitude modulation in the rf oscillator is detected to provide a control output signal.
Single-frequency keyable control systems suffer from the fact that a simple detection device discloses to a tamperer the frequency at which he must operate to actuate the unlocking mechanism. In fact, a tuneable absorption wavemeter, which is the simplest type of frequency measuring device would itself activate the pure adsorption unlocking mechanism in U.S. Pat. No. 3,723,967. A frequency system, operating at two or more frequencies simultaneously or in sequence, although increasing the difficulty, similarly suffers from the ability of a tamperer to detect the operating frequencies.